Rapid Bedrock Uplift in the Antarctic Peninsula Explained by Viscoelastic Response To

1 Rapid bedrock uplift in the Antarctic Peninsula explained by viscoelastic response to 2 recent ice unloading 3 4 Grace A. Nielda*, Valentina R. Barlettab, Andrea Bordonic, Matt A. Kingd,a, Pippa L. 5 Whitehousee, Peter J. Clarkea, Eugene Domackf, Ted A. Scambosg, Etienne Berthierh 6 7 aSchool of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, 8 UK. 9 bDTU Space, Technical University of Denmark, Lyngby, Denmark. 10 cDTU Physics, Technical University of Denmark, Lyngby, Denmark; DTU Compute, 11 Technical University of Denmark, Lyngby, Denmark. 12 dSchool of Land and Food, University of Tasmania, Hobart, Australia. 13 eDepartment of Geography, Durham University, Durham, UK. 14 fDepartment of Geoscience, Hamilton College, 198 College Hill Road, Clinton New York, 15 USA 13323; now at: College of Marine Science, University of South Florida, St. Petersburg, 16 Florida, USA. 17 gNational Snow and Ice Data Center, CIRES, University of Colorado, Boulder, Colorado, 18 USA. 19 hCentre National de la Recherche Scientifique, LEGOS, Université de Toulouse, Toulouse, 20 France. 21 *Corresponding author. E-mail address: [email protected]. Tel.:+44 (0)191 222 22 6323. 23 1 24 25 Abstract 26 Since 1995 several ice shelves in the Northern Antarctic Peninsula have collapsed and 27 triggered ice-mass unloading, invoking a solid Earth response that has been recorded at 28 continuous GPS (cGPS) stations. A previous attempt to model the observation of rapid uplift 29 following the 2002 breakup of Larsen B Ice Shelf was limited by incomplete knowledge of 30 the pattern of ice unloading and possibly the assumption of an elastic-only mechanism. We 31 make use of a new high resolution dataset of ice elevation change that captures ice-mass loss 32 north of 66°S to first show that non-linear uplift of the Palmer cGPS station since 2002 33 cannot be explained by elastic deformation alone. We apply a viscoelastic model with linear 34 Maxwell rheology to predict uplift since 1995 and test the fit to the Palmer cGPS time series, 35 finding a well constrained upper mantle viscosity but less sensitivity to lithospheric thickness. 36 We further constrain the best fitting Earth model by including six cGPS stations deployed 37 after 2009 (the LARISSA network), with vertical velocities in the range 1.7 to 14.9 mm/yr. 38 This results in a best fitting Earth model with lithospheric thickness of 100–140 km and 39 upper mantle viscosity of 6 × 1017 – 2 × 1018 Pa s - much lower than previously suggested for 40 this region. Combining the LARISSA time series with the Palmer cGPS time series offers a 41 rare opportunity to study the time-evolution of the low-viscosity solid Earth response to a 42 well-captured ice unloading event. 43 44 Key Words: Antarctic Peninsula, Larsen B, ice-mass loss, viscoelastic uplift, GPS, upper 45 mantle viscosity. 46 47 2 48 1. Introduction 49 Rapid changes in climate in the Antarctic Peninsula (AP) over the past 50 years have led 50 to the retreat and eventual collapse of several major ice shelves (Fig. 1), such as Prince 51 Gustav and Larsen A in 1995 (Rott et al., 1996), and Larsen B in 2002 (Rack and Rott 2004) 52 (see Cook and Vaughan (2010) for a complete summary). In response to ice shelf collapse, 53 tributary glaciers have exhibited acceleration and thinning (e.g., De Angelis and Skvarca 54 2003; Rignot et al., 2004; Scambos et al., 2004) and this dynamic ice loss induces a solid 55 Earth response which may be observed in GPS records. 56 The study of Thomas et al. (2011) identified markedly-increased uplift in GPS coordinate 57 time series from the Northern Antarctic Peninsula (NAP) that they associated with ice 58 unloading related to the breakup of Larsen B Ice Shelf in 2002. This uplift was best captured 59 in the near-continuous cGPS record at Palmer station which exhibited an increase in uplift 60 rate from 0.1 mm/yr prior to 2002.2, to 8.8 mm/yr thereafter. Thomas et al. (2011) suggested 61 that the effect was due to the elastic response of the solid Earth but they were not able to 62 satisfactorily reproduce the increased uplift rates with an elastic model, which they suggested 63 was at least partly due to the weakly defined magnitude and spatial pattern of ice-mass loss in 64 their model. 65 The NAP lies in a complex tectonic setting which passes from active subduction along the 66 South Shetland Trench, located north of the South Shetland Islands, to passive margin west of 67 65°W at the intersection of the Hero Fracture Zone with the Shetland Platform (see Fig. S1 in 68 the supplementary material). The Bransfield Basin separates the South Shetland Islands from 69 the NAP and has active volcanism along a mid-axial region, suggesting a back arc tectonic 70 setting (Barker and Austin 1998; Barker et al., 1991). The conversion from active subduction 71 to passive margin, and hence mantle conditions more likely reflective of low viscosity, took 72 place relatively recently at ~4 Ma before present along the AP margin just south of Hero 3 73 Fracture Zone (Barker et al., 1991). One of the GPS stations used in this study (ROBI, see 74 Section 2.1) lies along a presumed incipient rift axis as expressed by the active volcanic chain 75 of the Seal Nunataks (González-Ferrán 1983). However, there is little known in relation to 76 the mantle or crustal configuration beneath the Seal Nunatak region. The reader is referred to 77 Barker (1982) and Larter and Barker (1991) for a full tectonic history of the region. Due to 78 the active tectonic setting of the region, the mantle is likely to have a relatively low viscosity 79 compared with other locations undergoing deformation in response to changes in ice-mass 80 e.g. East Antarctica or Fennoscandia. Using a combination of inferred ice history, GPS and 81 GRACE data, Ivins et al. (2011) suggested this region has a relatively thin lithosphere (20- 82 45 km) and a low viscosity mantle (3-10 × 1019 Pa s). Due to the low viscosity nature of the 83 upper mantle, the Earth’s viscous response to ice-mass change in the AP is much more rapid 84 than in other regions of Antarctica, and post-1995 unloading events may hence be 85 contributing to the observed uplift in the NAP through viscoelastic rebound. Likewise, 86 assuming a Maxwell rheology, there may be very little, to no, residual response of the NAP 87 to unloading events associated with recession from the Last Glacial Maximum. 88 In this study we use cGPS data from the NAP to constrain a local model of solid Earth 89 response to a high resolution present-day mass loss field (Scambos et al., 2014, in review). 90 By comparing the modelled elastic uplift time series with the observed cGPS uplift time 91 series from Palmer Station we show that elastic uplift alone cannot reproduce the cGPS 92 observations. A viscoelastic model is then used to predict uplift based on a range of Earth 93 models, and results are compared with the Palmer record to obtain the range of best fitting 94 models. Finally, the Earth model is further refined using six shorter cGPS time series from 95 the NAP (see Table 1). 96 97 2. Data 4 98 2.1. GPS 99 Fig. 1 shows the locations of available cGPS sites in the NAP. Of these, we used the 100 seven sites closest to the region of ice-mass change (see Table 1). Palmer is a long term 101 station with an excess of 15 years of data, and the remaining six sites were installed in 2009- 102 2010 as part of the LARISSA project (LARsen Ice Shelf System, Antarctica) 103 (http://www.hamilton.edu/expeditions/larissa). We did not use the record from O’Higgins (a 104 compilation of three records from two adjacent stations, OHIG, OHI2, OHI3; labelled OHI2 105 on Fig. 1) as a constraint as it lies far from the region of largest mass loss and as such may be 106 affected by potential lateral heterogeneity in Earth structure. Spring Point (Bevis et al., 2009) 107 (SPPT on Fig. 1) was also excluded due to the lack of data at this site; however we compare 108 our results with both of these records in the supplementary material. 109 The cGPS data from 1998 through to the end of 2012 were processed using a Precise 110 Point Positioning strategy (Zumberge et al., 1997) within GIPSY-OASIS v6.1.2. 111 Homogeneously reprocessed (as of 2012) satellite clocks and fiducial-free orbits were fixed 112 to values provided by the Jet Propulsion Laboratory. The receiver clocks, tropospheric zenith 113 wet delay, tropospheric gradients and station coordinates were estimated in standard ways 114 (e.g. Thomas et al., 2011). For the troposphere, we adopted the Vienna Mapping Function v1 115 (Boehm et al., 2006) and we assumed hydrostatic zenith delays based on the European Centre 116 for Medium-Range Weather Forecasts. Higher order ionospheric effects (Petrie et al., 2011) 117 were not corrected. Solid Earth tides were corrected according to the IERS 2010 conventions 118 (Petit and Luzum 2010) and ocean tide loading displacements were corrected based on the 119 TPXO7.2 ocean tide model (Egbert et al., 2009), which has been shown to perform very well 120 in this region (King et al., 2011), using the SPOTL software (Agnew 1996; Agnew 1997) in a 121 centre of mass of the whole Earth system (CM) reference frame (Fu et al., 2012).

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